Support universal binaries on macOS

[AlexDenisov]

Currently, mull-cxx will extract doubled bitcode (e.g. for x96 and arm64) and mull-runner fails to read mutants from universal binary.

Steps to reproduce:

$ echo "int main(int argc) { return argc + argc; }" > test.c
$ clang -arch arm64 -arch x86_64 -g -fembed-bitcode test.c
$ mull-runner-12 a.out
[error] Cannot create SymbolicFile from: /private/tmp/sc-V1LsVrXo3/a.out
[error] Error messages are treated as fatal errors. Exiting now.
$ mull-cxx-12 a.out
[info] Link mutated program (threads: 1)
[error] Cannot link program
status: Failed
time: 100ms
exit: 70
command: clang /var/folders/1s/hps840w156xfn_m__h2m17880000gp/T/mull-d7e587.bc /var/folders/1s/hps840w156xfn_m__h2m17880000gp/T/mull-fdb944.bc -o /var/folders/1s/hps840w156xfn_m__h2m17880000gp/T/mull-168cac.exe
stdout:
stderr: warning: overriding the module target triple with x86_64-apple-macosx11.0.0 [-Woverride-module]
'apple-m1' is not a recognized processor for this target (ignoring processor)
'+crc' is not a recognized feature for this target (ignoring feature)
<truncated>

[benthevining]

Is there any news on this?

If not universal binaries, does/will mull ship binaries for Mac arm64? The current release is only x86_64, so trying to pass the plugin to clang on my M1 Mac fails because clang apparently can't open the plugin.

Do I need to build mull from source myself?

[AlexDenisov]

Hi @benthevining, thank you for bringing this up.

If not universal binaries, does/will mull ship binaries for Mac arm64?

We don't ship universal binaries yet, but there is no technical limitation in doing so. I'll see if I can cut a new release soon.

Do I need to build mull from source myself?

I think it is the best option to build mull on your own on the new Apple Silicon. Currently, we do no sign binaries on macOS, so even if we ship a universal binary, you'd then have to codesign it locally (perhaps with an ad-hoc certificate).

[AlexDenisov]

@benthevining I realized in the end why we don't do universal binaries: we are relying on LLVM from homebrew which is not universal itself, so we can't easily make mull a universal binary as well.

Until #957 is not resolved, I'll update the docs suggesting/warning about precompiled macOS binaries on Apple Silicon hardware.

[benthevining]

ohhh I see. That's a shame.

I would really appreciate some docs on building mull for M1, since I haven't been able to get it working (maybe because of the Homebrew LLVM issue?)

[AlexDenisov]

@benthevining, what doesn't work for you on M1? I'm using M1 as my main development machine, so I can certainly say that building Mull works there. But perhaps the instructions are missing something important.

[AlexDenisov]

I ended up adding support for universal binaries by teaching mull to only consider one slice from the universal binary. The slice is chosen on which version of mull-runner is running (e.g. arm64 or x86_64).
The changes are here.

@benthevining it doesn't address your use case as it's slightly different from the topic of this issue. If you still have issues with Mull on Apple Silicon - please, consider opening a separate issue.

[benthevining]

Thanks! I'll try again on my M1 machine and let you know if I have any issues.